Many modern radio frequency (RF) communication standards and specifications (in particular, RF communication standards and specifications involving carrier aggregation (CA)) involve RF communication bands for RF signals that span relatively large and high frequency ranges (often over 1.5 GHz). For example, these RF communication bands may span large frequency ranges because various channels of operation are defined within the RF communication band. Typically these RF communication bands are referred to as widebands. To operate in accordance with RF communication standards and specifications for these widebands, RF communication circuitry requires RF filters capable of providing passbands that maintain high passband integrity at these high frequencies through the entire frequency range of the wideband. Furthermore, the frequency separation between these widebands is often small. For example, a frequency range of the RF communication band B66 goes up to 2.2 GHz while a frequency range of the RF communication band B40 goes down to 2.3 GHz thereby resulting in a frequency separation of just 100 MHz between the RF communication band B40, B66. Thus, not only do these widebands require RF filters with passbands that do not introduce significant insertion losses throughout the entirety of these very high and very large frequency ranges, wide band RF communication standards and specifications also require that the passbands provide sharp roll off at the edges of the passband in order to reject noisy RF signals in very close adjacent RF communication bands. but spanning the entire wideband without introducing significant losses a insertions losses in These RF communication bands are typically referred to a widebands and may define various channels of operation within the RF communication band combinations involve at least one wide-frequency band. An acoustic filter with a wide bandwidth needs a high coupling coefficient and thus results in large flyback out of band and thus is hard to use for CA applications (due to insufficient rejection in the close band).
Traditional RF filters use networks of inductors and capacitive elements that define passbands to cover RF communication bands and stopbands to reject out of band noise. Unfortunately, traditional RF filters are not capable of providing passbands that maintain band integrity throughout the span of the frequency due to their quality (Q) factor limitations and are not capable of simultaneously providing the roll-off required for out-of-band rejection to filter out of can achieve a wide bandwidth (use large coupling coefficients) without giving significant flyback, which is advantageous for CA applications. However, LC filters have limited resonator quality factor, Q, of around a few hundreds, and thus cannot achieve a fast roll-off such as the roll-off needed to multiplex closely spaced frequency bands such as B66-B40 that are 100 MHz apart).
Instead, modern RF technology instead typically employs acoustic wave filters. Acoustic wave filters are often formed by a network of acoustic wave resonators. Acoustic wave filters are capable of providing passbands with much better band integrity throughout the span of the wideband. In particular, bulk acoustic wave resonators (BARs) are often employed to form acoustic wave filters that provide passbands that maintain band integrity thought the span of widebands with frequency ranges above 1.5 GHz. To reject noise from adjacent widebands, the acoustic wave resonators in the acoustic wave filter are arranged so that a stopband is defined from roll off from a band edge of the passband. In this manner, the stopband is defined adjacent to the passband to provide out of band rejection. However, in order for these acoustic wave filters to provide passbands that can maintain integrity throughout the passband, the acoustic wave resonators of the acoustic wave filter need to have high coupling coefficients. The high coupling coefficients between the acoustic wave resonators unfortunately result in the stopbands having high flyback. Often, this high flyback prevents the stopband from providing adequate out of band rejection. Furthermore, fabrication variations and changes in filter behavior due to operating conditions (e.g., variation in filter behavior due to the operating temperature) often result in misalignments that can result in unnecessary insertion losses within the passband or inadequate out of band rejection by the stopband.
Currently, hard tuning techniques, such as mechanically trimming the layers associated with the bulk acoustic wave resonators, are employed to calibrate the acoustic wave filter and prevent these misalignments. However, not only do mechanical trimming techniques permanently alter the acoustic wave resonators, but they often require acoustic wave resonator topologies which are spatially inefficient due to the need to maintain access to the trimmable features of the acoustic wave resonators. Furthermore, it is difficult to employ these mechanical trimming techniques with the accuracy needed to prevent or correct misalignments at the edges of the passbands when the frequency displacement of the wideband and adjacent RF communication bands is so small.